The symptoms of a cold and the flu can often look similar, but there are some key differences.

Seasonal allergies are a third variable to consider.

The only way to get properly diagnosed is with a doctor’s visit. Knowing what you have can help determine the best treatment.

It can start with a sniffle. But before your symptoms turn into full-blown illness, you want to know: is it a cold or the flu?

Both illnesses can share a variety of symptoms. They’re also both caused by viruses. And, of course, there’s a third potential culprit to consider: allergies.

Most diagnostic detective work is best left to a doctor. But in the meantime, there are a few variables to consider that can help you narrow in on the cause of your illness.

The chief symptom of a cold is a stuffy or runny nose

If your nose feels stuffed or runny – but not itchy – and you don’t have a fever, it’s likely that you’ve been hit by a cold. Paying attention to the time of year is helpful, too. During the fall, roughly 75% of all circulating viruses are rhinoviruses, the most common cause of colds.

Here’s the good news: most colds only last three to seven days, though symptoms can linger for another week or so.

To start feeling better, avoid stocking up on vitamin C supplements, which studies suggest won’t do much for your symptoms. Instead, try a zinc lozenge – some research indicates zinc may help shorten the duration of a cold by interfering with the way rhinoviruses replicate.

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If it’s allergies, you’ll probably be itchy and sneezy

Seasonal allergies, which typically crop up in the spring and summer, are also accompanied by nasal symptoms, but they differ in several key ways from those that characterize a cold, according to the National Institutes of Health. You’ll usually have an itchy – as opposed to a runny – nose, and your eyes will likely feel scratchy too. You’ll also probably be sneezing.

Your doctor can tell you if you suffer from an allergy and prescribe the right treatment for you. If you’re suffering from allergies, symptoms can last as long as whatever allergen triggering them remains in the air, which can be more than a month.

The flu is the worst of the three

Flu season strikes from late fall to late winter, typically peaking in February. The first thing you should do if you suspect you’ve come down with the flu is to take your temperature. Most flu cases are accompanied by a fever of more than 100 degrees Fahrenheit. Flu sufferers also typically experience body aches, coughs, and extreme tiredness, according to the Centers for Disease Control and Prevention. In people who are especially vulnerable, like older people, children, and individuals with weakened immune systems, the virus can also cause serious complications like pneumonia.

The flu can stick around in your system for longer than a cold, with symptoms like fatigue persisting up to three weeks. Individuals infected with the flu can pass it to anyone within 6 feet, and only stop being contagious once they’ve been fever-free for a full 24 hours (without the help of medication).

If you’re already sick with the flu, make sure you rest, since sleep is key to a properly functioning immune system, and keep an eye on your symptoms to be sure you don’t develop more severe complications.

If you’re not sick yet, the best defense against the virus is the flu shot. Getting vaccinated can also make the flu less miserable if you do get it.